The present invention relates to an adaptive control unit for an electrical discharge machine, and more particularly to an adaptive control unit of an electrical discharge machine which maintains the machine in a desired machining state at the start of electrical discharge machining.
FIG. 13 illustrates a conventional adaptive control unit for an electrical discharge machine, which is disclosed in Japanese Patent Publication No. 10769-1987. The electrical discharge machine includes a machining electrode 1, a workpiece 2, a machining tank 3 filled with a dielectric 4, a spindle 5 for moving the machining electrode 1 in a Z direction, a drive motor 6 for driving the spindle 5, a speed/position detector 7 for detecting the travel speed and position of the spindle 5, an electrode position control section 21 for controlling the position of the machining electrode 1 by providing a drive command to the drive motor 6, a machining power supply 22 for supplying a machining voltage across the machining electrode 1 and workpiece 2, and a detected value processor 23, responsive to a detection signal from the position detector 7 and a machining gap voltage, for providing a feedback command to the electrode position control section 21 and to the machining power supply 22, and for providing an electrode bottom point raising command or machining command to an adaptive control section 31. The adaptive control section 31 transmits a machining command to the electrode position control section 21 and machining power supply 22 in accordance with the incoming command signal from the detected value processor 23.
In operation, a pulse-shaped voltage is applied across the machining electrode 1 and workpiece 2 by the machining power supply 22 causing an electrical discharge to occur therebetween in the dielectric. The workpiece 2 is machined by the electrical discharge and the feed operation of the machining electrode 1. The electrode position control section 21 compares an average machining gap voltage provided by the detected value processor 23 with a reference voltage to maintain a predetermined machining gap between the machining electrode 1 and workpiece 2 for electrical discharge, and controls the drive motor 6 in accordance with the differential voltage (i.e., the difference between the average machining gap voltage and the reference voltage) to control the position or feed rate of the machining electrode 1.
The gap between the machining electrode 1 and workpiece 2 is generally in the range of about ten microns to several tens of microns. When the area to be machined is wide, it is difficult to remove the chips produced by the machining through this gap which, in turn, causes the chips to reside in the machining gap. Moreover, the amount of chips generated is more than what typically can be removed. As a result, a faulty electrical discharge (e.g., electrical discharge concentrating in one area) is more likely to occur. This, however, can be prevented by detecting a faulty state and suppressing the amount of chips being produced or removing more chips.
FIGS. 14A and 14B illustrate the movement of the machining electrode 1, wherein FIG. 14A shows the movement of the electrode during normal machining, and FIG. 14B illustrates the electrode movement where a fault has occurred in the machining gap.
In the machining process, the machining electrode 1 oscillates over a distance of about several microns to several tens of microns. During a normal machining operation, the electrode bottom point 101 (i.e., where the machining electrode 1 switches from lowering to rising), gradually lowers in the course of machining. If a fault occurs in the machining gap, however, the electrode bottom point 101 tends to rise as shown in FIG. 14B. Hence, to prevent the rise of the electrode bottom point 101 and to suppress the amount of chips being produced, the width of the current pulse supplied by the machining power supply 22 is decreased, and the electrical discharge dwell width is increased. In order to remove more chips, the regular rising distance of the machining electrode 1 can be increased to increase the "pumping" action created by electrode movement and consequent dielectric flow.
Referring back to FIG. 13, the detected value processor 23 detects the electrode bottom point 101 in accordance with the motion of the machining electrode 1 provided by the position detector 7 and puts out a signal to the adaptive control section 31 that indicates whether the electrode bottom point 101 is rising or lowering. When the bottom point rises above a predetermined threshold value, the adaptive control section 31 determines that a fault has occurred in the machining gap and transmits a command (e.g., for decreasing the current pulse width and increasing the electrical discharge dwell width so as to suppress the amount of chips being produced or for raising the machining electrode 1 so as to enhance the chip removing capability), to the electrode position control section 21 and machining power supply 22.
From the above description, it should be apparent that shortening the current pulse width or increasing the regular raising amount of the machining electrode 1 is necessary to prevent a faulty electrical discharge from occurring.
A second conventional electrical discharge machine is described in Japanese Patent Disclosure Publication No. 107831-1981. This machine accurately determines the machining gap state during machining using an electrode position detecting means, which detects a difference between the deepest position of the electrode relative to the workpiece and the current electrode position. A machining gap state determining means detects an increase in the difference as a faulty machining gap state and outputs a signal accordingly.
As described above, the adaptive control unit of a conventional electrical discharge machine determines that a fault has occurred in the machining gap when the electrode rises or when the difference between the deepest position of the electrode fed to the workpiece and the current position thereof has exceeded a predetermined threshold value. The adaptive control unit then reduces the current pulse width and elongates the electrical discharge width to suppress the stock removal ability (i.e., in other words, the amount of chips being produced) or to raise the electrode for enhancing the chip removing capability.
When electrical discharge machining is performed with a large machining electrode (i.e., when the area to be machined is large, about 25 cm.sup.2 or greater, at the start of electrical discharge machining), a conventional control unit can detect the occurrence of an arc phenomenon and conduct avoidant control of machining conditions. But the conventional device will sometimes erroneously determine that the arc phenomenon has occurred and perform avoidance control of the machining conditions, which reduces machining efficiency. That is, the machining time is increased by the operation of elongating the electrical discharge dwell width, reducing the electrode down time, increasing the electrode raising amount, etc. The arc phenomenon is liable to occur particularly when a machining electrode, made of copper, is used for electrical discharge machining without flushing dielectric through the machining gap.
In addition, when a large machining electrode is used for electrical discharge machining, the known art determines that the electrical discharge machining state is faulty for a certain period of time and continues to perform avoidant control of the machining conditions because the difference between the deepest position of the electrode fed to the workpiece and the current position thereof increases at the start of electrical discharge machining. The difference between the deepest and current positions of the electrode is not eliminated immediately due to the large area to be removed by electrical discharge machining.